1. Field of the Invention
The present invention relates to an optical disc apparatus capable of accessing an optical disc recording medium at a high speed and with a high precision and a method of tracking control of the same.
2. Description of the Related Art
In an optical disc apparatus performing the recording and/or reproduction of data with respect to an optical disc such as a compact disc (CD), a seek operation for accessing a desired track is usually carried out by a combination of a rough seek operation for moving an optical pick-up by a sled actuator and a fine seek operation for moving an object lens by the actuator in the optical pick-up.
In the seek operation, first, the optical pick-up is moved by the sled actuator and stopped in the vicinity of a target address. Next, the track pull-in and tracking operation are carried out and the address information reached is read. Then, a fine seek operation, for example, repetition of a step jump (one-track jump) for crossing one track, is carried out to move the object lens of the optical pick-up to the desired track.
When the sled on which the optical pick-up is mounted is moved at a high speed by the seek operation as mentioned above, as shown in FIG. 1A, the object lens is initially left behind the movement of the sled by the force of inertia and there after vibrates. This causes disadvantages. For example, it prevents the reduction and stabilization of settling time and causes deterioration of optical characteristics due to movement of the object lens toward standard scope. Therefore, as shown in FIG. 1B, servo control of the center point of the object lens is frequently carried out to hold the object lens at the center of the field at the time of movement of the sled by using a center point error signal output in proportion to the movement of the object lens toward the scope.
However, when the optical pick-up is stopped after moving the sled, even if center point servo control is carried out as mentioned above, the object lens will vibrate and also the sled itself will vibrate, thus the tracking error signal will vibrate. If the frequency of vibration of this tracking error signal is high, the track pull-in cannot be properly carried out. For this reason, as shown in FIGS. 2A to 2C, after a certain time T elapses from when the sled movement is ended, the mode of control is switched from the center point servo control to the tracking servo control, the track pull-in is carried out, and the tracking servo control is applied. This waiting time T prior to switching is set to, e.g., several milliseconds.
However, the frequency of vibration of the tracking error signal at the end of the movement of the sled as explained above and the time until the vibration sufficiently subsides change considerably depending on various conditions, for example, the eccentricity of the disc. For this reason, the time T until the mode of control is switched from the center point servo control to the tracking servo control as mentioned above, is set to a time sufficiently long enough to cope even with even vibration which takes a long time to subside. As a result, a longer than necessary settling time must often be ensured, so there arises the disadvantage that the speed of accessing a desired track becomes slow. Further, even if a certain long time is set, the possibility of occurrence of a vibration that requires a longer setting time cannot be completely eliminated. Where such a vibration occurs, there also arises a disadvantage in that the track pull-in cannot be adequately carried out and track accessing becomes unstable.